Composite material phase change memory cell

ABSTRACT

A phase change memory (PCM) cell includes a first electrode comprised of a first electrically conductive material, a second electrode comprised of a second electrically conductive material, and a phase change section positioned between the first electrode and the second electrode. The phase change section includes a first phase change material having a first resistance drift coefficient, and a second phase change material having a second resistance drift coefficient that is greater than the first resistance drift coefficient. An axis of the PCM cell extends between the first electrode and the second electrode, and the second phase change material is offset from the first phase change material in a direction that is perpendicular to the axis.

BACKGROUND

The present invention relates to computer memory, and more specifically,to phase change material memory devices with composite phase changematerials.

Phase change memory (PCM) can be utilized for both training andinference in analog computing for artificial intelligence. The PCMstructures can include phase change memristive devices with tunableconductivities and overall high device resistance with high retention tominimize energy consumption. The tuning can be accomplished by formingdifferent structural states with varying proportions of crystalline andamorphous phases of PCM material. However, PCM materials can suffer fromresistance drift over time, which can negatively affect the fidelity ofthe tuning.

SUMMARY

According to an embodiment of the present disclosure, a PCM cellincludes a first electrode comprised of a first electrically conductivematerial, a second electrode comprised of a second electricallyconductive material, and a phase change section positioned between thefirst electrode and the second electrode. The phase change sectionincludes a first phase change material having a first resistance driftcoefficient, and a second phase change material having a secondresistance drift coefficient that is greater than the first resistancedrift coefficient. An axis of the PCM cell extends between the firstelectrode and the second electrode, and the second phase change materialis offset from the first phase change material in a direction that isperpendicular to the axis.

According to an embodiment of the present disclosure, a method ofmanufacturing a PCM cell includes forming a first electrode, forming aphase change section electrically connected to the first electrode, andforming a second electrode on the phase change section. The phase changesection includes a first phase change material having a first resistancedrift coefficient, and a second phase change material having a secondresistance drift coefficient that is greater than the first resistancedrift coefficient.

According to an embodiment of the present disclosure, a PCM cellincludes a first electrode comprised of a first electrically conductivematerial, a second electrode comprised of a second electricallyconductive material, and a phase change section positioned between thefirst electrode and the second electrode. The phase change sectionincludes a first phase change material having a first resistivity, and asecond phase change material having a second resistivity that is greaterthan the first resistivity. An axis of the PCM cell extends between thefirst electrode and the second electrode, and the second phase changematerial is offset from the first phase change material in a directionthat is perpendicular to the axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-section view of a mushroom PCM cell with a compositephase change section in a polycrystalline configuration, in accordancewith embodiments of the present disclosure.

FIG. 1B is a cross-section view of the mushroom PCM cell with thecomposite phase change section including an amorphous zone, inaccordance with embodiments of the present disclosure.

FIG. 2 is a graph of resistance drift coefficient versus logarithmicresistance between the set and reset phases for various PCM cellconfigurations, in accordance with embodiments of the presentdisclosure.

FIG. 3 is a flowchart of a method of manufacturing the PCM cell of FIG.1A, in accordance with embodiments of the present disclosure.

FIGS. 4A-4G are a series of cross-section views of the method of FIG. 3of manufacturing the PCM cell, in accordance with embodiments of thepresent disclosure.

FIG. 5 is a flowchart of a method of manufacturing the PCM cell of FIG.1A, in accordance with embodiments of the present disclosure.

FIGS. 6A-6G are a series of cross-section views of the method of FIG. 5of manufacturing the PCM cell, in accordance with embodiments of thepresent disclosure.

FIGS. 7A-7H are cross-section views of alternate embodiment mushroom PCMcells, in accordance with embodiments of the present disclosure.

FIGS. 8A and 8B are cross-section views of alternate embodiment confinedPCM cells, in accordance with embodiments of the present disclosure.

FIG. 9 is a cross-section view of alternate embodiment pillar PCM cell,in accordance with embodiments of the present disclosure.

FIG. 10A is a cross-section view of an alternate embodiment bridge PCMcell, in accordance with embodiments of the present disclosure.

FIG. 10B is a top view of bridge PCM cell of FIG. 10A, in accordancewith embodiments of the present disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of the present disclosure. Itis noted that various connections and positional relationships (e.g.,over, below, adjacent, etc.) are set forth between elements in thefollowing description and in the drawings. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present disclosure is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layers “C”and “D”) are between layer “A” and layer “B” as long as the relevantcharacteristics and functionalities of layer “A” and layer “B” are notsubstantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus. Inaddition, any numerical ranges included herein are inclusive of theirboundaries unless explicitly stated otherwise.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements. It should benoted, the term “selective to,” such as, for example, “a first elementselective to a second element,” means that a first element can beetched, and the second element can act as an etch stop.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography.

Deposition can be any process that grows, coats, or otherwise transfersa material onto the wafer. Available technologies include physical vapordeposition (PVD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), molecular beam epitaxy (MBE) and more recently, atomiclayer deposition (ALD) among others. Another deposition technology isplasma enhanced chemical vapor deposition (PECVD), which is a processwhich uses the energy within the plasma to induce reactions at the wafersurface that would otherwise require higher temperatures associated withconventional CVD. Energetic ion bombardment during PECVD deposition canalso improve the film's electrical and mechanical properties.

Removal/etching can be any process that removes material from the wafer.Examples include etch processes (either wet or dry), chemical mechanicalplanarization (CMP), and the like. One example of a removal process ision beam etching (IBE). In general, IBE (or milling) refers to a dryplasma etch method which utilizes a remote broad beam ion/plasma sourceto remove substrate material by physical inert gas and/or chemicalreactive gas means. Like other dry plasma etch techniques, IBE hasbenefits such as etch rate, anisotropy, selectivity, uniformity, aspectratio, and minimization of substrate damage. Another example of a dryremoval process is reactive ion etching (RIE). In general, RIE useschemically reactive plasma to remove material deposited on wafers. WithRIE the plasma is generated under low pressure (vacuum) by anelectromagnetic field. High-energy ions from the RIE plasma attack thewafer surface and react with it to remove material.

Semiconductor doping can be the modification of electrical properties bydoping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (“RTA”).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device.

Semiconductor lithography can be the formation of three-dimensionalrelief images or patterns on the semiconductor substrate for subsequenttransfer of the pattern to the substrate. In semiconductor lithography,the patterns are formed by a light sensitive polymer called aphotoresist. To build the complex structures that make up a transistorand the many wires that connect the millions of transistors of acircuit, lithography and etch pattern transfer steps are repeatedmultiple times. Each pattern being printed on the wafer is aligned tothe previously formed patterns and gradually the conductors, insulatorsand selectively doped regions are built up to form the final device.

FIGS. 1A and 1B are cross-section views of PCM cell 100 for use in, forexample, an integrated circuit (not shown). In the illustratedembodiment, PCM cell 100 comprises bottom wire 102, bottom electrode104, insulator 106, heater 108, insulator 110, projection liner (PL)112, PCM section 114 (which includes a first, undoped material 116 and asecond, doped material 118), insulator 120, top electrode 122, and topwire 124.

In the illustrated embodiment, the bottom of bottom electrode 104 is indirect contact with and electrically connected to the top of bottom wire102, which can receive electrical signals from other components (notshown) of the integrated circuit. The bottom of heater 108 is in directcontact with and electrically connected to the top of bottom electrode104. The bottom of PL 112 is in direct contact with and electrically andthermally connected to the top of heater 108. The bottom of PCM section114 is in direct contact with and electrically and thermally connectedto the top of PL 112. The bottom of top electrode 122 is in directcontact with and electrically connected to the top of PCM section 114.The bottom of top wire 124 is in direct contact with and electricallyconnected to the top of top electrode 122, and top wire 124 can deliverelectrical signals from PCM cell 100 to other components (not shown) ofthe integrated circuit.

In the illustrated embodiment, insulators 106, 110, 120 structurallysupport and electrically isolate the other components of PCM cell 100,selectively, and fill in the space therebetween, as appropriate. Thus,the outer side of bottom electrode 104 is in direct contact with andlaterally surrounded by insulator 106, and the outer side of heater 108is in direct contact with and laterally surrounded by insulator 110.Furthermore, the bottom side of PL 112 is in direct contact with andaxially adjacent to insulator 110, and top wire 124 are in directcontact with and laterally surrounded by insulator 120, and topelectrode 122 is in direct contact with and laterally and axiallyadjacent to insulator 120.

In the illustrated embodiment, a cross-section of PCM cell 100 (into thepage in FIG. 1 ) can be circular, although in other embodiments, it canbe rectangular, square, oval, or any other suitable shape. In addition,the widths of PCM section 114, PL 112, and top electrode 122 are thesame, whereas the width of heater 108 is substantially reduced,comparatively (e.g., three to seven times smaller, or about five timessmaller). Thereby, PCM cell 100 can be said to have a mushroomconfiguration wherein an electrical signal (i.e., electrical current)can flow from bottom electrode 104 to top electrode 122 through heater108, PL 112, and PCM section 114. In some embodiments, the width ofheater 108 is between 35 and 40 nanometers (nm), and the widths of PL112, PCM section 114, and top electrode 122 are between 100 nm and 400nm. PCM section 114 has a height of about 80 nm, and the width ofundoped material 116 within PCM section 114 is between 30 nm and 60 nm.Thus, in some embodiments, the width of doped material 118 can beone-and-a-half to fourteen times larger than undoped material 116, andin some embodiments, the width of doped material 118 can be three toseven times larger than undoped material 116. Furthermore, in someembodiments, the width of undoped material 116 can be three-quarters totwice the size of heater 108.

In the illustrated embodiment, heater 108 and undoped material 116 arepositioned centrally along axis 128 of PCM cell 100 that extends betweenbottom electrode 104 and top electrode 122 (which is parallel to thedirection of current flow in PCM cell 100 during operation). Asdepicted, axis 128 may extend longitudinally through PCM cell 100 suchthat axis 128 extends through each of top wire 124, top electrode 122,undoped material 116, heater 108, bottom electrode 104, and bottom wire102. In some embodiments, PCM cell 100 may be mirrored in at least somedirections across axis 128. Doped material 118 surrounds undopedmaterial 116, and doped material 118 is offset from axis 128 in alateral direction (i.e., perpendicular to axis 128). Thereby, undopedmaterial 116 is positioned closer to heater 108 than doped material 118is.

In the illustrated embodiment, bottom electrode 104 and top electrode122 are comprised of a very electrically conductive material, such asmetal or metallic compound, for example, titanium nitride (TiN) ortungsten (W). Heater 108 is an electrode that is comprised of TiN or ahigher resistance metal, such as, for example, titanium tungsten (TiW),tantalum nitride (TaN), or titanium aluminide (TiA1), and has arelatively narrow cross-sectional area, which focuses electrical currentthat is run through PCM cell 100. This allows heater 108 to generateheat through resistive heating during a pulse of electricity, which canbe used to selectively change the temperature of PCM section 114, forexample, above the crystallization temperature and the meltingtemperature of undoped material 116 and doped material 118. In addition,heater 108 can be comprised of multiple different electricallyconductive materials that can be arranged in multiple layers.

In the illustrated embodiment, insulators 106, 110, 120 are comprised ofa dielectric (electrical insulating) material, such as, for example,silicon nitride (SiN), silicon oxide (SiO₂), silicon nitride carbide(SiNC), or tetraethyl orthosilicate (TEOS). In some embodiments, all ofthe insulators 106, 110, 120 are the same material, and in otherembodiments, different materials are used for some or all of insulators106, 110, 120. In addition, PL 112 is comprised of a moderatelyelectrically resistive material, such as a metal and/or semiconductor(e.g., TaN; tungsten nitride (WN); amorphous carbons (a-C); doped a-C;transparent conductive oxides such as tin-doped indium oxide (ITO),aluminum zirconium oxide (AZO), and high-resistance metal chalcogenides(ex. titanium selenide (TiSe)), and other poorly conducting metalnitrides). The material comprising PL 112 have a higher electricalresistivity than polycrystalline phases of undoped material 116 anddoped material 118 but lower electrical resistivity than amorphousphases undoped material 116 and doped material 118.

In the illustrated embodiment, undoped material 116 is composedessentially of a phase change material such as agermanium-antimony-tellurium (GST), gallium-antimony-tellurium (GaST),or silver-iridium-antimony-telluride (AIST) material, although othermaterials can be used as appropriate. Examples of other PCM materialscan include, but are not limited to, germanium-tellurium compoundmaterial (GeTe), silicon-antimony-tellurium (Si—Sb—Te) alloys,gallium-antimony-tellurium (Ga—Sb—Te) alloys,germanium-bismuth-tellurium (Ge—Bi—Te) alloys, indium-tellurium (In—Se)alloys, arsenic-antimony-tellurium (As—Sb—Te) alloys,silver-indium-antimony-tellurium (Ag—In—Sb—Te) alloys, Ge—In—Sb—Tealloys, Ge—Sb alloys, Sb—Te alloys, Si—Sb alloys, Ge—Te alloys andcombinations thereof. The terms “composed essentially” and “consistessentially,” as used herein with respect to materials of differentlayers, indicates that other materials, if present, do not materiallyalter the basic characteristics of the recited materials. For example,an undoped material 116 consisting essentially of GST material does notinclude other materials that materially alter the basic characteristicsof the GST material.

On the other hand, doped material 118 can be a mixture of a phase changematerial (e.g., similar to or the same as undoped material 116) and aphase separated dopant material such as, for example, one or moredielectric materials and/or poorly-electrically conductive materials(e.g., oxygen (O), nitrogen (N), carbon (C), SiO2, SiO, SiON, SiOC,tantalum nitride (Ta3N5), aluminum nitride (AlN), and titanium nitride(TiN)). The grains of the phase separated dopant material can restrictthe grain size of the phase change material and provide “nano opens”(i.e., local regions of relatively high electrical resistance) toincrease the resistance of doped material 118, and, in some embodiments,the amount of the phase separated dopant material doped material 118 isat least 10% by volume. Doped material 118 can also be a substitutionalor interstitial doped phase change material such as, for example,titanium-GST (TiGST), gallium-GST (GaGST), silicon-GST (SiGST), orbismuth-GST (BiGST), since these atoms can substitute/sit on intersticesdue to their solubility in GST.

In the illustrated embodiment, PCM cell 100 can be operated as a memorycell by passing an electrical current pulse from bottom electrode 104 totop electrode 122 to program PCM cell 100. This can be done at a varietyof voltages and/or for a variety of durations to read or write a valueon PCM cell 100. For example, to write, a high voltage can be used(e.g., 1 volt (V) to 4 V) for a short duration, which can cause heater108 to locally heat PCM section 114 beyond the melting points of undopedmaterial 116 and doped material 118. Once the flow of current ceases,PCM section 114 can cool down rapidly, which forms amorphous zone 126 ina process called “resetting”. Zone 126 is a dome-shaped region of PCMsection 114 having an amorphous configuration (in both undoped material116 and doped material 118), although the remainder of PCM section 114is still in a polycrystalline configuration. In general, this amorphousconfiguration has no definite structure. However, there can be local,disjoint crystalline nuclei (i.e., small, crystallized regions of phasechange section 114) present in zone 126. The creation of zone 126 cancause the electrical resistance across PCM cell 100 to increase ascompared to a solely polycrystalline configuration (a la PCM cell 100 inFIG. 1A). These resistance values of PCM cell 100 can be read withoutchanging the phase of PCM section 114 (including that of zone 126) orthe resistance value of PCM cell 100, for example, by sending a currentpulse at a low voltage (e.g., 0.2 V) from bottom electrode 104 to topelectrode 122.

In addition, PCM section 114 can be rewritten and returned back to asolely polycrystalline configuration by “setting” PCM cell 100. One wayto rewrite PCM section 114 uses a high voltage electrical pulse (e.g., 1V to 4 V) for a short period of time (e.g., 10 nanoseconds (ns)), whichcan cause PCM section 114 to heat up beyond the crystallization pointsof undoped material 116 and doped material 118 but not to their meltingpoints. Since the crystallization temperature is lower than the meltingtemperature, once the flow of current ceases, PCM section 114 can annealand form crystals. Another way to rewrite PCM section 114 uses anelectrical pulse with a relatively long trailing edge (e.g., 1microsecond) (as opposed to a square pulse with a relatively shorttrailing edge on the order of nanoseconds) that is strong enough to heatPCM section 114 beyond the melting points of undoped material 116 anddoped material 118, after which, PCM section 114 is cooled down slowly,allowing crystals to form. Either of these processes cause theelectrical resistance across PCM cell 100 to decrease as compared tohaving an amorphous zone 126. This new resistance value can then be readusing current at a low voltage (e.g., 0.2 V) without changing the phaseof PCM section 114 or the resistance value of PCM cell 100.

In some embodiments, the melting temperatures of undoped material 116and doped material 118 can be around 600° C. In some embodiments, thecrystallization temperatures of undoped material 116 and doped material118 can be around 180° C. In addition, the process of setting andresetting PCM cell 100 can occur repeatedly, and in some embodiments,different zones 126 with different resistances can be created in PCMmaterials 114 (e.g., due to having different sizes of zone 126 and/oramounts of crystallization nuclei in zone 126). This allows for PCM cell100 to have various distinct resistances that can be created by varyingthe resetting parameters. Thereby, if PCM cell 100 is considered torepresent information digits, these digits can be non-binary (as opposedto traditional bits). However, in some embodiments, PCM cell 100 can beused as a bit by either having or not having a uniform zone 126 in PCMsection 114. In such embodiments, PCM cells 100 can have a highresistance (a.k.a., low voltage output or “0”) or low resistance(a.k.a., high voltage output or “1”).

While undoped material 116 and doped material 118 can functionsimilarly, their properties can vary from one another. For example,undoped material 116 can have a lower (electrical) resistance driftcoefficient (in both the amorphous phase and the polycrystalline phase)and a lower electrical resistance compared to doped material 118, whichhas a higher (electrical) resistance drift coefficient (in both theamorphous phase and the polycrystalline phase) and a higher electricalresistance. However, the resistance drift coefficients and electricalresistances of both undoped material 116 and doped material 118 arelower in the polycrystalline set phases than in the amorphous resetphases, respectively. Therefore, the resistance drift coefficient ofdoped material 118 in the set phase can be lower than the resistancedrift coefficient of undoped material 116 in the reset phase. Similarly,the resistance of doped material 118 in the set phase can be lower thanthe resistance of undoped material 116 in the reset phase.

The result of these properties is shown in FIG. 2 , which is graph 200of resistance drift coefficient versus resistance between the set andreset phases for various PCM cell configurations. In the illustratedembodiment, resistance drift coefficient is displayed using a linearscale whereas resistance is displayed using a logarithmic scale. Thethree illustrated PCM cell configurations are homogenous PCM materialwithout a projection liner, homogenous PCM material with a projectionliner, and composite PCM section 114 with PL 112 (i.e., PCM cell 100).For each configuration, there is a line that extends from a set state(without an amorphous zone 126) to a reset state (includes a largeamorphous zone 126). In the illustrated embodiment, a PCM cell withhomogenous PCM material but no projection liner has low resistance andlow drift in the set state, and high resistance and high drift in thereset state. A PCM cell with homogenous PCM material and a projectionliner has moderate resistance and moderate drift in the set state, andhigh resistance and low-moderate drift in the reset state. However, PCMcell 100 has low-moderate resistance and low drift in the set state, andhigh resistance and low-moderate drift in the reset state. Therefore, onaverage, the resistance drift of PCM cell 100 is less than the other twoconfigurations.

This phenomenon can be explained by the configuration of PCM cell 100.When all of PCM section 114 is in the polycrystalline set phase, currentwill primarily travel from heater 108 to top electrode 122 via undopedmaterial 116 due to its lower resistance. Since the resistance driftcoefficient of undoped material 116 in the set phase is low, the overalldrift of PCM cell 100 will also be low. However, when there is asignificant amorphous zone 126 (shown in FIG. 1 ), the current canspread out in PL 112 to avoid zone 126 due to its high resistance.Instead, the current will primarily travel to top electrode 122 viadoped material 118. Although the resistance drift coefficient of dopedmaterial 118 is higher than that of undoped material 116 (in the samephases), most of doped material 118 is in the polycrystalline set phase(as opposed to the amorphous reset phase of a substantial portion ofundoped material 116). Thereby, the overall drift of PCM 100 will below-moderate. In addition, the span of resistance between the set stateof PCM cell 100 and the reset state of PCM cell 100 is lengthened, whichindicates a broader dynamic (i.e., usable) range of resistances. Thisincreases the resolution of PCM cell 100.

FIG. 3 is a flowchart of method 300 of manufacturing PCM cell 100. FIGS.4A-4G are a series of cross-section views of method 300 of manufacturingthe PCM cell. FIGS. 3 and 4A-4G will now be discussed in conjunctionwith one another wherein each operation of method 300 is illustrated byone of FIGS. 4A-4G. In addition, during this discussion, references maybe made to features of PCM cell 100 shown in FIGS. 1A-2 , however, somefeatures may be omitted for the sake of simplicity (e.g., bottom wire102, bottom electrode 104, insulator 106, and top wire 124).

In the illustrated embodiment, method 300 starts at operation 302,wherein insulating layer 330 is formed on bottom electrode 104 andinsulator 106. At operation 304, a via is formed in insulating layer330, for example, using etching to form insulator 110. Then the via isfilled to form heater 108. At operation 306, PL 112 is formed on heater108 and insulator 110, and doped layer 332 is formed on PL 112. Atoperation 308, via 334 is formed in doped layer 332 to expose PL 112 andform doped material 118. At operation 310, undoped layer 336 is formedon PL 112 and doped material 118. At operation 312, chemical mechanicalpolishing (CMP) is performed to remove the excess undoped PCM materialto form undoped material 116 that is coterminous with doped material 118(thus forming PCM section 114). Then, top electrode 122 is formed ondoped material 116 and undoped material 118. At operation 314,insulating layer 338 is formed on top electrode 122.

The components, configuration, and operation of PCM cell 100 and method300 allow for PCM section 114 to have a composite configuration that iscomprised of two different PCM materials (i.e., undoped material 116 anddoped material 118). Thereby, PCM section 114 includes different regionsthat have different material properties.

FIG. 5 is a flowchart of method 400 of manufacturing PCM cell 100. FIGS.6A-6G are a series of cross-section views of method 400 of manufacturingthe PCM cell. FIGS. 5 and 6A-6G will now be discussed in conjunctionwith one another wherein each operation of method 400 is illustrated byone of FIGS. 6A-6G. In addition, during this discussion, references maybe made to features of PCM cell 100 shown in FIGS. 1A-2 , however, somefeatures may be omitted for the sake of simplicity (e.g., bottom wire102, bottom electrode 104, insulator 106, and top wire 124).

In the illustrated embodiment, method 400 starts at operation 402,wherein insulating layer 440 is formed on bottom electrode 104 andinsulator 106. At operation 404, a via is formed in insulating layer438, for example, using etching to form insulator 110. Then the via isfilled to form heater 108. At operation 406, PL 112 is formed on heater108 and insulator 110, and undoped layer 442 is formed on PL 112. Atoperation 408, undoped material 116 is formed from undoped layer 442,which exposes PL 112. At operation 410, doped layer 444 is formed on PL112 and undoped material 116. At operation 412, chemical mechanicalpolishing (CMP) is performed to remove the excess doped PCM material toform doped material 118 that is coterminous with undoped material 116(thus forming PCM section 114). Then, top electrode 122 is formed ondoped material 116 and undoped material 118. At operation 414,insulating layer 446 is formed on top electrode 122.

The components, configuration, and operation of PCM cell 100 and method300 allow for PCM section 114 to have a composite configuration that iscomprised of two different PCM materials (i.e., undoped material 116 anddoped material 118). Thereby, PCM section 114 includes different regionsthat have different material properties.

FIGS. 7A-7H are cross-section views of alternate embodiment mushroom PCMcells 500A-500H, respectively. In the illustrated embodiment of PCM cell500A, undoped material 516A tapers towards PL 512A. Among other methods,PCM cell 500A can be manufactured using method 300 (shown in FIG. 3 ).

In the illustrated embodiment of PCM cell 500B, undoped material 516Btapers towards top electrode 522B. Among other methods, PCM cell 500Bcan be manufactured using method 400 (shown in FIG. 5 ).

In the illustrated embodiment of PCM cell 500C, undoped material 516Ctapers towards the middle, creating an hourglass shape. Among othermethods, PCM cell 500C can be manufactured by forming the bottom and tophalves of PCM section 514C separately. For example, operations 402-410,and the CMP portion of 412 can be performed to form the bottom half ofPCM section 514C. Then, operations 306-314 can be performed to form thetop half of PCM section 514C and complete PCM cell 500C.

In the illustrated embodiment of PCM cell 500D, undoped material 516Dtapers from the middle, creating a diamond shape. Among other methods,PCM cell 500D can be manufactured by forming the bottom and top halvesof PCM section 514D separately. For example, operations 302-310, and theCMP portion of 312 can be performed to form the bottom half of PCMsection 514D. Then, operations 406-414 can be performed to form the tophalf of PCM section 514D and complete PCM cell 500D.

In the illustrated embodiment of PCM cell 500E, undoped material 516Etapers towards PL 512E and only extends halfway through PCM section 514Etherefrom. Among other methods, PCM cell 500E can be manufactured byforming the bottom and top halves of PCM section 514E separately.

In the illustrated embodiment of PCM cell 500F, undoped material 516Ftapers towards top electrode 522F and only extends halfway through PCMsection 514F therefrom. Among other methods, PCM cell 500F can bemanufactured by forming the bottom and top halves of PCM section 514Fseparately.

In the illustrated embodiment of PCM cell 500G, PCM section 514G has agraded transition between the centrally-located undoped material 516Gand the peripherally-located, fully doped material 518G. In other words,the amount of dopant material in PCM section 514G gradually increasesfrom the center (which has none) to the lateral exterior (which has thefull concentration of dopant material). This is in contrast to theabrupt transitions between the undoped material and the doped materialpresent in PCM cells 100, 500A-500F, and 500H-500L. In thoseembodiments, the amount of dopant material changes in a virtuallydiscontinuous manner since they include composite PCM sections comprisedof a portion of undoped PCM material (that is itself homogenous) incontact with a portion of fully-doped PCM material (which is itselfhomogenous but different from the undoped portion). Among other methods,PCM cell 500G can be manufactured by performing operations 402-406.Then, a block mask is placed over the center of undoped layer 442 (whichwill become undoped material 516G), and ion implant of dopant isperformed (creating doped material 518G). In some embodiments, undopedmaterial 516G is actually not doped, and in some embodiments, undopedmaterial 516G is less doped than doped material 518G.

In the illustrated embodiment of PCM cell 500H, PCM cell 500H is similarto PCM cell 100 (shown in FIG. 1A). However, PCM cell 500H lacks aprojection liner.

FIGS. 8A and 8B are cross-section views of alternate embodiment confinedPCM cells 500I and 500J, respectively. In the illustrated embodiment ofPCM cell 500I, PCM cell 500I includes bottom electrode 504I, PL 512I,PCM section 514I (which includes undoped material 516I and dopedmaterial 518I), and top electrode 522I. While PCM cell 500I has aconfined cell configuration, it still has doped material 5181surrounding undoped material 5161 such that doped material 518I isoffset from axis 528I in a lateral direction (i.e., perpendicular toaxis 5281).

In the illustrated embodiment of PCM cell 500J, PCM cell 500J is similarto PCM cell 500I. However, undoped material 516J is in direct contactwith PL 512J instead of having doped material 518J therebetween.

FIG. 9 is a cross-section view of alternate embodiment pillar PCM cell500K. In the illustrated embodiment of PCM cell 500K, PCM cell 500Kincludes bottom electrode 504K, PL 512K, PCM section 514K (whichincludes undoped material 516K and doped material 518K), and topelectrode 522K. While PCM cell 500K has a pillar cell configuration, itstill has doped material 518K surrounding undoped material 516K suchthat doped material 518K is offset from axis 528K in a lateral direction(i.e., perpendicular to axis 528K).

FIG. 10A is a cross-section view of an alternate embodiment bridge PCMcell 500L, and FIG. 10B is a top view of bridge PCM cell 500L. In theillustrated embodiment of PCM cell 500L, PCM cell 500L includes firstelectrode 504L, PL 512L, PCM section 514L (which includes undopedmaterial 516L and doped material 518L), and second electrode 522L. PCMcell 500K has a bridge cell configuration such that first electrode 504Lis coplanar with second electrode 522L. Thereby, PL 512L and PM section514L extend parallel to axis 528L (which extends between electrodes 504Land 522L), instead of being centered on axis 528L. During operation ofPCM cell 500L, current flows perpendicular to axis 528L proximate to thewide electrodes 504L and 522L. However, as the current crosses thenarrow bridge 548L, axis 528L is still parallel to the direction ofcurrent flow. Because the active portion of PM section 514L (i.e., theportion where amorphous zone 126 would be created and eliminated) islocated in bridge 548L, PCM cell 500L still has doped material 518Lsurrounding undoped material 516L such that doped material 518L isoffset from the flow of current in a lateral direction (i.e.,perpendicular to axis 528L).

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A phase change memory (PCM) cell comprising: afirst electrode comprised of a first electrically conductive material; asecond electrode comprised of a second electrically conductive material;and a phase change section positioned between the first electrode andthe second electrode, the phase change section comprising: a first phasechange material having a first resistance drift coefficient; and asecond phase change material having a second resistance driftcoefficient that is greater than the first resistance drift coefficient;wherein an axis of the PCM cell extends between the first electrode andthe second electrode; and wherein the second phase change material isoffset from the first phase change material in a direction that isperpendicular to the axis.
 2. The PCM cell of claim 1, wherein the phasechange section has a graded transition between the first phase changematerial and the second phase change material.
 3. The PCM cell of claim1, wherein the phase change section has an abrupt transition between thefirst phase change material and the second phase change material.
 4. ThePCM cell of claim 1, wherein the first phase change material is undopedand the second phase change material is doped.
 5. The PCM cell of claim1, further comprising a projection liner between the first electrode andthe phase change section, wherein the projection liner is comprised of amaterial with a resistivity between a resistivity of a polycrystallinephase of the first phase change material and a resistivity of anamorphous phase of the first phase change material.
 6. The PCM cell ofclaim 1, further comprising a heater positioned between the firstelectrode and the phase change section.
 7. The PCM cell of claim 6,wherein: the heater has a first width; the phase change section has asecond width; and the second width is greater than or equal to thricethe first width.
 8. The PCM cell of claim 6, further comprising aprojection liner between the heater and the phase change section,wherein the projection liner is comprised of a material with aresistivity between a resistivity of a polycrystalline phase of thefirst phase change material and a resistivity of an amorphous phase ofthe first phase change material.
 9. A method of manufacturing a phasechange memory (PCM) cell, the method comprising: forming a firstelectrode; forming a phase change section electrically connected to thefirst electrode, wherein the phase change section comprises: a firstphase change material having a first resistance drift coefficient; and asecond phase change material having a second resistance driftcoefficient that is greater than the first resistance drift coefficient;and forming a second electrode on the phase change section.
 10. Themethod of claim 9, further comprising forming a projection liner betweenthe first electrode and the phase change section.
 11. The method ofclaim 9, further comprising forming a heater between the first electrodeand the phase change section.
 12. The method of claim 11, wherein: theheater has a first width; the phase change section has a second width;and the second width is greater than or equal to thrice the first width.13. The method of claim 9, wherein forming the phase change sectioncomprises: depositing the second phase change material; etching thesecond phase change material to form a via; and filling the cavity withthe first phase change material.
 14. The method of claim 9, whereinforming the phase change section comprises: depositing the first phasechange material; etching the first phase change material to form apillar; and surrounding the pillar with the second phase changematerial.
 15. The method of claim 9, wherein forming the phase changesection comprises grading the phase change section to transition fromthe first phase change material to the second phase change material. 16.The method of claim 9, wherein the first phase change material isundoped and the second phase change material is doped.
 17. A phasechange memory (PCM) cell comprising: a first electrode comprised of afirst electrically conductive material; a second electrode comprised ofa second electrically conductive material; and a phase change sectionpositioned between the first electrode and the second electrode, thephase change section comprising: a first phase change material having afirst resistivity; and a second phase change material having a secondresistivity that is greater than the first resistivity; wherein an axisof the PCM cell extends between the first electrode and the secondelectrode; and wherein the second phase change material is offset fromthe first phase change material in a direction that is perpendicular tothe axis.
 18. The PCM cell of claim 17, wherein: the first phase changematerial has a first resistance drift coefficient; and a second phasechange material having a second resistance drift coefficient that isgreater than the first resistance drift coefficient.
 19. The PCM cell ofclaim 17, further comprising a heater positioned between the firstelectrode and the phase change section.
 20. The PCM cell of claim 17,further comprising a projection liner between the first electrode andthe phase change section, wherein the projection liner is comprised of amaterial with a resistivity between a resistivity of a polycrystallinephase of the first phase change material and a resistivity of anamorphous phase of the first phase change material.